Optimized ActivityManager.getCurrentUser()

That method returns the userId of the current foreground user, but its
implementation fetches a UserInfo then extracts id.

This CL shortcuts the process by returning the user id directly, which
improves the performance in 2 ways:

- Avoids a map lookup.
- Decrease data pased through the IPC.

Test: adb shell am get-current-user
Fixes: 151322436

Change-Id: I028a54a1918bd2226ed3bd903bbe98c6ef4811b2